topic Re: MSM765 not routing DHCP request back to client in M and MSM Series

MSM765 not routing DHCP request back to client

kminihane — Thu, 24 Mar 2011 20:36:31 GMT

I have an MSM765 in my core switch, configured as a WLAN controller for both my corporate WiFi and a Guest WLAN. My guest WLAN is configured to route out to a DSL router and the MSM765 has dhcp-relay configured to point to the LAN interface on the DSL router (IP address 192.168.1.1)

My guest WLAN is configured on VLAN 80, while my corporate traffic is configured on VLAN 7. The internet port on my guest VLAN (80) is configured with an IP address of 192.168.1.250

A client requesting a DHCP address from the DSL router is unable to obtain an address. Upon investigation (I put a packet sniffer on the network) I discovered that the DSL router is receiving the DHCP request from the guest WLAN (192.168.1.250) but is trying to send the response to 192.168.10.1 which is a configured LAN address, as per the attached screen grab.

Any ideas?

I'd appreciate any help on this.

Re: MSM765 not routing DHCP request back to client

Fredrik Lönnman — Fri, 25 Mar 2011 06:34:41 GMT

Hi,

I think Ive seen in some doc that the DHCP-server has to be reachable over the LAN-interface. In your packet sniff, do you see where the first DHCPDISCOVER originates from? Is it unicasted from 192.168.10.1?

Re: MSM765 not routing DHCP request back to client

kminihane — Tue, 29 Mar 2011 10:18:21 GMT

Thanks for the reply. In my packet capture, I've noticed that the original DHCP Discover packet appears to come from my Internet Port address (192.168.1.250). However, when I drill down into the packet, I then notice that the relay agent address comes up as the LAN port IP address (192.168.10.1)

I am able to ping the DHCP server from the WLAN controller

Thanks for any help

Re: MSM765 not routing DHCP request back to client

Ulrich Saur — Mon, 11 Jul 2011 13:49:49 GMT

We resolved this by adding a special route entry on the DHCP server which routes the DHCP-requested subnet to the LAN port of the controller. Doesn't sound nice but works...